1. Field of the Invention
The present invention relates in general to a system for separating luminance and chrominance signals from a composite video signal in a television broadcasting system, such as a NTSC system, a PAL system or a SECAM system, and more particularly to an improved system for separating luminance and chrominance signals from a composite video signal, which is suitable to obtain a better quality of picture in high vision systems of high resolution, such as a high definition television set (HDTV), an enhanced definition television set (EDTV) and a high definition video cassette recorder (HDVCR).
2. Description of the Prior Art
FIG. 1 is a block diagram of a conventional system for separating luminance and chrominance signals from a composite video signal utilizing both frame and line comb filters and FIG. 2 is a block diagram of another conventional system for separating luminance and chrominance signals from a composite video signal utilizing only a line comb filter and having a construction different from the system in FIG. 1. Herein, a frame represents one picture of a television broadcasting signal. In a television broadcasting system such as, for example, a NTSC system, each of frames, or pictures consists of 525 lines, which is sent every 1/30 sec from a broadcasting station. In each of the frames, odd and even lines, respectively of 252.5 lines are in turn sent every 1/60 sec from the broadcasting station in a standard interlaced scanning manner. From this fact, it can be seen that each of the frames consists of the two fields. In the NTSC system, each of the lines is provided in the unit of 15.75 KHz (64.5 .mu.sec). Each of frame signals to be used below indicates a video signal which is delayed by 525 lines as mentioned above, i.e., a video signal next to two fields or 1/30 sec and a frame difference signal is obtained by subtracting a frame signal prior to one frame from the current frame signal or subtracting the current frame signal from a frame signal next to one frame. This frame difference signal is comprised of only a chrominance signal component due to removal of a luminance signal component in the composite video signal, as will be described later.
The basic principles of the comb filters used in the systems of FIGS. 1 and 2 will now be described with reference to FIGS. 3A and 3B.
FIG. 3A illustrates a one-dimensional frequency spectrum showing an arrangement of the luminance and chrominance signals every line in the television broadcasting signal in the NTSC system and FIG. 3B illustrates a one-dimensional frequency spectrum showing an arrangement of the luminance and chrominance signals every frame in the television broadcasting signal in the NTSC system. The television broadcasting signal in the NTSC system is in the mixed form of the chrominance and luminance signals which are superposed on each other on the frequency spectra as shown in FIGS. 3A and 3B. The comb filter is provided to separate the chrominance and luminance signals from the composite video signal. In the television broadcasting signal in the NTSC system, the chrominance signal is 180.degree. out of phase every line and, also, every frame (every 525 lines). As a result, since two successive lines of the chrominance signal are oppositely phased with each other, a comb filtered luminance signal output is provided by combining two successive lines of the composite video signal together additively, resulting in cancellation of the chrominance signal. On the contrary, since lines of the luminance signal are originally in phase, a comb filtered chrominance signal output is provided by combining two successive lines of the composite video signal together subtractively, resulting in cancellation of the luminance signal. Therefore, the comb filters in the systems of FIGS. 1 and 2 can separate the luminance and chrominance signals from the composite video signal utilizing the above principles.
First, the operation of a construction in FIG. 1 will be described.
Upon input of a composite video signal CV, a frame comb filter 1 separates luminance and chrominance signals from the composite video signal CV in the unit of frame and then applies the separated frame chrominance signal FC to an operator 4. At the same time, the composite video signal CV is also inputted by a line comb filter 2. The line comb filter 2 separates luminance and chrominance signals from the composite video signal CV in the unit of line and then applies the separated line chrominance signal LC to an operator 5.
The frame comb filter 1 also applies a frame difference signal FD to a moving level detector 3 for detection of a moving level of the video signal. The moving level detector 3 takes a moving factor K corresponding to the moving level of the video signal, based on the frame difference signal FD, and then feeds the taken moving factor K to the operators 4 and 5. Herein, the moving factor K may be any value between 0 and 1. Namely, 0.ltoreq.K.ltoreq.1. In this connection, it is noted that the more the moving factor K is approximate to 1, the more the moving level of the video signal becomes severe.
The operator 4 multiplies the received frame chrominance signal FC by a value (1-K) which is obtained by subtracting the moving factor K from 1 and the operator 5 multiplies the received line chrominance signal LC by the moving factor K. Output signals from the operators 4 and 5 are together added in an adder 6, which then outputs the added chrominance signals to a band pass filter 8.
In the television broadcasting signal in the NTSC system, the chrominance signal is typically placed at a frequency band of .+-.0.5 MHz around a center frequency of 3.58 MHz, as can be seen on a standard frequency spectrum. In this connection, outputted from the band pass filter 8 is the final chrominance signal C of a predetermined frequency band of about 3 MHz-4.2 MHz. The final chrominance signal C from the band pass filter 8 is applied to a subtracter 9.
The composite video signal CV is also inputted by a delay element 7 in which the composite video signal CV is then delayed by a predetermined period of time for synchronization with the chrominance signal C output from the band pass filter 8.
The subtracter 9 subtracts the chrominance signal C from the predetermined time delayed composite video signal CV and then outputs the resulting luminance signal as the final luminance signal Y.
Then, the operation of a construction in FIG. 2 will be described.
Upon input of a composite video signal CV, a line comb filter 11 separates luminance and chrominance signals from the composite video signal CV in the unit of line and then applies the separated line chrominance signal LC to a band pass filter 12.
The band pass filter 12 functions to extract a desired chrominance signal component from the received line chrominance signal LC. As a result, outputted from the band pass filter 12 is only a desired component of chrominance signal C of a frequency band of .+-.0.5 MHz around a center frequency fsc.
The line chrominance signal LC from the line comb filter 11 is also inputted by a low pass filter 13 which is provided to remove a chrominance subcarrier from the line chrominance signal LC. For this reason, outputted from the low pass filter 13 is only the residual luminance signal component .DELTA.Y in which no chrominance signal component is present.
Since the chrominance and luminance signals are overlapped with each other as shown in FIG. 3A, the line comb filter 11 provides the comb filtered or separated line chrominance signal LC in which the residual luminance signal component .DELTA.Y may be included. Noticeably, the residual luminance signal component .DELTA.Y is increased more and more for a moving video signal. The residual luminance signal component .DELTA.Y from the low pass filter 13 is applied to an absolute value generator 14, which then generates an absolute value of the received residual luminance signal component .DELTA.Y. This absolute value is applied to a non-inverting input terminal (+) of a comparator CP1, an inverting input terminal (-) of which is applied with a predetermined threshold value Vth1. The comparator CP1 compares the absolute value at its non-inverting input terminal (+) with the predetermined threshold value Vth1 at its inverting input terminal (-) and outputs the resultant value as a switching control signal to a switch SW1.
The composite video signal CV is also inputted by a delay element 15 in which the composite video signal CV is then delayed by a predetermined period of time for synchronization with the chrominance signal C output from the band pass filter 12. A subtracter 16 subtracts the chrominance signal C from the predetermined time delayed composite video signal CV from the delay element 15 and then feeds the resulting luminance signal to a delay element 17 and a band trap filter 18.
The switch SW1 is connected to the output of the delay element 17 at its one input terminal b and to the output of the band trap filter 18 at its other input terminal b such that it selectively outputs one of the two inputs as the final luminance signal Y, in response to the switching control signal from the comparator CP1. That is, if the switching control signal from the comparator CP1 is high, resulting from the presence of a dot interference, the switch SW1 is activated to select the output of the band trap filter 18. On the contrary, with the switching control signal from the comparator CP1 being low, the switch SW1 is activated to select the output of the delay element 17. The signal selected in the switch SW1 in this manner is outputted as the final luminance signal Y.
The above-mentioned conventional systems for separating the luminance and chrominance signals from the composite video signal utilizing the line and frame comb filters are shown in IDTV Receiver (Kiyoyuki Kawai et al., Toshiba Corporation Consumer Products Engineering Laboratory Yokohama, Japan, IEEE Transactions on consumer Electronics, Vol. CE-33, No. 3, August 1987).
In accordance with the construction in FIG. 2, however, the conventional system utilizing only the line comb filter has a disadvantage, in that cross-talk components may occur because of imperfect separations at vertical boundaries since the current line signal, a signal prior to one line and a signal next to one line which are outputted from the line comb filter are filtered directly by the band pass filter and the low pass filter, respectively. The cross-talk components result in occurrence of a dot interference exerting an adverse effect on the quality of picture.
Also in accordance with the construction in FIG. 1, the conventional system utilizing both the line and frame comb filters has a disadvantage, in that moving of the chrominance signal or the high frequency signal cannot be detected since the moving level detector 3 in the system detects the moving level of the video signal depending on a signal (i.e., the residual luminance signal component) which is obtained by low pass-filtering the frame difference signal FD.